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We present a spectroscopic determination of the redshift of the second sourcein the Jackpot gravitational lens system J0946+1006, for which only aphotometric estimate of $z_{\rm phot}$ = 2.41$^{+0.04}_{-0.21}$ has previouslybeen available. By visually inspecting an archival VLT X-Shooter observation,we located a single emission line from the source in the H-band. Among thepossible options we find that this line is most likely to be [OIII] 5007 Ang at$z_{\rm spec} $ = 2.035. Guided by this proposal, we were able to detect thefaint CIII] 1907,1909 Ang emission doublet in a deep VLT MUSE datacube. TheCIII] emission is spatially coincident with the brightest parts of the secondEinstein ring, and strongly supports the redshift identification. Thespectroscopic redshift is only marginally consistent with the photometricestimate. Re-examining the cosmological constraints from J0946+1006, therevised measurement favours less negative values of the dark energyequation-of-state parameter $w$; when combined with a cosmic microwavebackground prior, we infer $w$ = $-1.04\pm0.20$. The revised redshift does notsignificantly help to reconcile the small discrepancy in the image positionsfor the even more distant third source in J0946+1006.

A fully-spectroscopic triple-source-plane lens: the Jackpot completed